The present invention relates to the field of growing plants.
For decades scientists have delved ever deeper into the inner workings of plants, and particularly into those processes which are driven by the chemical capture of light energy. At the same time, research into new methods for converting electricity into light of particular wavelengths has led some engineers to try to produce artificial lighting which promotes plant growth. Until recently this has meant modifying energy inefficient “white light” sources to produce more light at wavelengths known to promote plant growth and health. This hybrid technology, in which the bulk of the light from these augmented “plant grow lights” can't be used efficiently by plants, has dominated the market for four decades.
While electricity was abundant and cheap, these “old school” plant grow lights, based mainly on HID, high pressure sodium, or fluorescent style lamps, were acceptable despite their imperfections. But they still have many shortcomings. They typically convert only 10-15% of electrical energy into light, and only a very small portion of that light can be used by plants. Some of them, particularly the HID lamps, emit short wavelength UV light which is damaging to both the plants being grown under them and the people tending the plants. All of these lamps generate waste heat which must be eliminated to prevent damage to the plants they illuminate, adding to their operational cost. They contain environmentally damaging metals, are fragile, and have a short operating life.
As electricity supplies fail to keep pace with demand, leading to ever higher prices, the need for more efficient plant growing lights increases. The latest generation of high output LEDs, with their narrow light output wavelengths, are a good choice for creating the next generation of plant grow lighting. Most LED plant grow lighting systems available today can only be used in a laboratory. The others, while claiming to be useful to commercial plant growers, are merely modifications of the laboratory-specific systems.
To our knowledge, no one has yet developed an efficient LED-based plant growing light that is amenable to both home lighting design and commercial plant production. By designing our LED lamp as a bulb, which can be used in industry standard lighting fixtures, we have created a product that has universal appeal and marketability. Our lamp can be manufactured inexpensively with readily available parts for both home and commercial use.
Its preferred power source is the subject of our copending utility patent application Ser. No. 10/397,763 filed Mar. 26, 2003 and entitled USE OF TRACK LIGHTING SWITCHING POWER SUPPLIES TO EFFICIENTLY DRIVE LED ARRAYS.
A key part of our research involved the determination of which light frequencies or wavelengths would produce superior plant growth results. Each plant pigment absorbs light at one or more specific wavelengths. The areas of peak absorption for each pigment are narrow, and the measurements made with pigments concentrated in a test tube are different than those done on living plants. The wavelength of the light used determines it's energy level, with shorter wavelengths having greater energy than longer wavelengths. Thus each absorption peak, measured by the wavelength of light at which it occurs, represents an energy threshold that must be overcome in order for the process to function.
There are many peaks of light absorption in the pigments found in plants, and ideally it would be best to match them each with the most appropriate LED. But this is not practicable because of the limited desired area available in the lamp being designed, and because LEDs are not available in every wavelength of the spectrum. The compromise is to see what LEDs are readily available and match them, as well as one is able, to groups of closely matched pigment absorption peaks, while striving to meet the minimum requirements of plants for healthy growth.
Our patent searches turned up U.S. Pat. Nos. 5,278,432 and 5,012,609, both issued to Ignatius et al., who suggest LED plant radiation very broadly within bands 620-680 or 700-760 nm (red) and 400-500 nm (blue). After a year and a half of research, we settled on three more specific light wavelengths that produced the best plant growth results.
660 nanometers (nm) is the wavelength that drives the engine of the photosynthetic process. The 680 nm wavelength is perhaps closer to the peak absorption wavelength of one of the two chlorophylls found in higher plants. However, at 680 nm you miss completely the absorption curve of the second chlorophyll, and furthermore the output curve of a 680 nm LED has a fair amount of light output above 700 nm, which is known to cause unwanted morphological changes to plants. LEDs of 680 nm output are also rare in the marketplace, making them relatively expensive. Our choice of a 660 nm first wavelength component is a compromise wavelength commonly used in plant growing research, which supplies energy to both types of chlorophyll without emitting enough light above 700 nm to adversely affect plant growth.
The 620 nm LEDs used in the aforesaid Ignatius et al. patents, are meant to provide the light energy for photosynthesis, but a look at the absorption spectrum for the two chlorophylls shows that this wavelength falls almost entirely outside the absorption curve for chlorophyll.
Our research showed better results using LEDs of 660 nm and 612 nm rather than the wavelengths of 620 nm and 680 nm. Beneficially, LEDs of 660 nm are also readily available in the market, and are very inexpensive.
Our second 612 nm wavelength component was selected not to promote photosynthesis, but to match one of the peaks of the carotenoids. As noted in “Influence of UV-B irradiation on the carotenoid content of Vitis vinifera tissues,” C. C. Steel and M. Keller (http://bst.portlandpress.com/bst/028/0883/bst028883.htm), “carotenoid synthesis . . . is dependent upon the wavelength of visible light, and is diminished under yellow and red filters.”
By providing the orange 612 nm light, we not only promote creation of carotenoids, which are required for plant health, but also add a little to photosynthesis, since the carotenoids pass their absorbed energy to chlorophyll. Carotenoids are required for plant health due to their ability to absorb destructive free radicals, both from solar damage and from chlorophyll production, whose precursors will damage plant tissue in the absence of the carotenoids. During research we found that, beneficially, test plants turned a deeper green, i.e. produced more chlorophyll, with the addition of our 612 nm light component. This ability to increase a plant's chlorophyll content with this specific light wavelength is an important aspect of our invention.
Blue light of about 465 nm, this wavelength being non-critical, is strongly absorbed by most of the plant pigments, but is preferably included as the third component in our lamp to support proper photomorphogenesis, or plant development. Any LED near this wavelength will work as well, but the 470 nm LEDs are commonly available and less expensive than many other blue LEDs.
Regarding the proper proportion for each wavelength, it is known, from independent laboratory research, that a blue/red proportion of 6-8% blue to red is optimal. In sunlight the blue/red light proportion is about 30%, but this is not required by plants. More than 8% blue light provides no additional benefit, but adds to the cost of the device since blue LEDs are among the most expensive to manufacture. In our device we include about 8% blue light, which is near optimal for plant development while offering the greatest cost savings. Our research showed that best results were obtained when the output of the 612 nm orange LEDs in our device was added to the output of the 660 nm red LEDs when calculating our most desired blue/red proportion.
Our lamp is intended to deliver a well mixed blend of all three of the wavelengths used to the plant it is illuminating. Other devices which are intended to grow plants with LEDs solve this problem by creating alternating rows of each wavelength of LED used, with each LED string being composed of LEDs of the same wavelength. In these other devices, though, the LEDs are arranged in a square or rectangular block, matching the shape of the device itself In our case, with a circular design, this is not the most effective way to align the LEDs.
To improve the manufacturability of our circular lamp, it proved better to use LED strings that mixed wavelength, i.e. instead of putting the 660 nm LEDs into their own strings, we use strings that contain both 660 nm and 612 nm LEDs, and in one string use all three wavelengths. Normally this isn't done because it offers a greater potential for having a “current hogging” LED alter the string's designed operating characteristics. Current hogs can be a problem even when all of the LEDs in a string are of the same wavelength and manufacture, but when the string is composed of a mixture of wavelengths the chances of having this problem are increased. LED strings of mixed wavelength are to be used when the supplied voltage and current is tightly controlled.
Regarding prior art found during our searches, the mounting and plug in of an LED array light module in a MR-16 or the like fixture is disclosed in Lys U.S. Pat. No. 6,340,868 in FIGS. 20 and 21. Lyes teaches the use of these LED array modules for accelerating plant growth; see FIGS. 92A and 92B. Lys also teaches in FIG. 22 the use of a 24 volt DC module for energizing three LED strings connected in parallel. Lowrey U.S. Pat. No. 6,504,301 discloses an MR-16 outline package for a mixed wavelength LED arrangement; other lighting packages such as MRC-11 etc. are mentioned in his specification col. 7. Okuno U.S. Pat. No. 4,298,869 discloses a conventional lamp screw in fixture for three parallel LED strings of two volt LEDs supplied by 19.5 volts. The concept of placing the LEDs very close to the plants as they generate little heat is taught in col. 1 of U.S. Pat. No. 6,474,838.